Seabed sediment, particularly in the Pacific Ocean, has recently been identified as having elevated concentrations of rare earth elements. See, Kato, Y., Fujinaga, Y., Nakamura, K., Takaya, Y., Kitamura, K., Ohta, J., Toda, R., Nakashima, T., and Iwamori, V., 2011; Deep-sea mud in the Pacific Ocean as a potential resource for rare-earth elements, Nature Geoscience, 4, 535-539, incorporated herein by reference. Indications are that the seabed sediments may be leached with a weak HCl of H2SO4 acidic solution. Once in solution the rare earth elements may potentially be recovered by a process of ion-exchange, then rare earth oxides may be formed by precipitation. Economic and engineering studies on the commercial recovery of the rare earth elements are limited and have mixed conclusions. See, Tetsuo Yamazaki, Yuta Yamamoto, Naoki Nakatani and Rei Arai, 2014, Preliminary Economic Evaluation of Deep-Sea REE Mud Mining, ASML: 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering OMAE2014-23141, incorporated herein by reference. Yamazaki proposed recovery of the sediment to a production support vessel followed by decanting and drying of the sediment for transfer to an onshore acid leach plant. Bashir et al proposed performing all the processing on the seabed. See, M Bashir, S H Kim, E Kiosidou, H Wolgamot, W Zhang, 2012, A Concept for Seabed Rare Earth Mining in the Eastern South Pacific, The Lloyd's Register Education Trust (LRET) Collegium 2012 Series, Volume 1, incorporated herein by reference. Yamazaki's economic assessment resulted in a negative internal rate of return based on 2012 REE prices and an assumed distribution of elements from samples taken in the Japanese exclusive economic zone. Bashir et al estimated costs for a 2000 tpy REE operation of CAPEX=$186 MM and OPEX of $61 MM/year. An internal rate of return was not computed but a range of NPVs were computed for different discount rates and different revenues based on low, present, and high average prices. Using a nominal rare earth element value of $80/kg an internal rate of return of 39% would be computed for the Bashir et al costs. On this basis seabed mining of the sediments would be economic. However, operation of a process on the seabed is not possible given the current state-of-the-art and is very risky.
The present invention is based upon processing of the sediment on board the production vessel to produce a mixed rare earth product. Economic studies have indicated this could be a superior approach in terms of return and lower risk than either of the methods described above.
A rare earth element (REE) or rare earth metal (REM), as defined by the International Union of Pure and Applied Chemistry, is one of a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides, as well as scandium and yttrium. Scandium and yttrium are considered rare earth elements because they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties. Rare earth elements are commonly categorized as “light” or “heavy” rare earths based on their atomic weight (LREE and HREE respectively). An additional category of “critical” rare earths (CREE) has been added by the U S Department of Defense to identify those of strategic importance to the defense industry. A final category is “essential” rare earth elements (FREE) which consist of all the elements including scandium and yttrium, but excluding relatively low value lanthanum, cerium, and praseodymium. Table 1 shows the categories of rare earth elements.
TABLE 1Categorization of rare earth elementsAtomic WeightSymbolLREEHREECREEEREE21Sc✓✓39Y✓✓✓57La✓58Ce✓59Pr✓✓60Nd✓✓✓61Pm✓✓62Sm✓✓63Eu✓✓✓64Gd✓✓65Tb✓✓✓66Dy✓✓✓67Ho✓✓68Er✓✓69Tm✓✓70Yb✓✓71Lu✓✓
There is no evidence of serious exploration of the seabed for a REE resource Kato et al analyzed a number of existing core samples. Good REF grades were found across the Pacific Ocean, especially in the South East Pacific. Anything above 1000 ppm would be considered significant. Higher REE concentrations in the upper five meters of the sediment, makes extraction of the rare earth elements easier.
Current REE samples are not close enough together to make any definitive resource estimate, but simply because of the number of samples with high concentrations and the vast area of the oceans it could be speculated that the quantity of rare earth elements available for mining would be lame.
While the overall grade of oceanic rare earth elements deposits is relatively low compared to some terrestrial deposits, there is a relatively high proportion of essential rare earth elements to total rare earth elements in the oceanic deposits. Higher ratios of essential rare earth elements mean that the processing and extraction of the rare earth elements is more efficient.
Additionally, oceanic REE deposits tend to have less radioactive products such as thorium and uranium which present formidable environmental challenges in mining of other rare earth deposits. Preliminary engineering and economic studies indicate that recovery of rare earth elements from seabed sediments may be economic.